The relationship between the location of a stimulus and activation of particular sensory neurons is a major theme in the field of sensory physiology. The receptive field of a sensory neuron is the region that, when stimulated, affects the activity of that neuron. For example, a sensory receptor might be activated by indentation of only a small area of skin. That area is the excitatory receptive field of the sensory receptor. Moreover, a neuron in the CNS might have a receptive field several times as large as that of a sensory receptor because it may receive information from many sensory receptors, each with a slightly different receptive field. The receptive field of that CNS neuron is thus the sum of the receptive fields of the sensory receptors that influence it. The location of the receptive field is determined by the location of the sensory transduction apparatus responsible for signaling information about the stimulus to the sensory neuron.
In general, sensory receptive fields are excitatory. However, a central sensory neuron can have either an excitatory or an inhibitory receptive field or, indeed, a complex receptive field that includes areas that excite it and areas that inhibit it. Examples of such complex receptive fields are discussed in
Coding of Information by Action Potentials
Central to CNS function is the transmission of information between neurons. This is accomplished primarily through action potentials, which propagate down the axon to the presynaptic terminals and cause neurotransmitter release, signaling the postsynaptic cells. As already explained, the regenerative nature of action potentials allows them to carry signals regardless of the length of the axon, whereas local signals, such as receptor or synaptic potentials (see ), decay with distance and are therefore not suitable for this purpose. The tradeoff, however, is that the all-or-none nature of action potentials means that their shape and size do not generally convey information in the way gradations of local potentials do. Instead, the variations in the rate or timing of action potentials appear to be used primarily as the “codes” for transmission of information between neurons.
Rate coding refers to information being coded in the firing rate of a neuron, where firing rate is defined as the number of spikes fired per unit time, usually expressed as spikes/second, also called hertz (Hz). For example, the force of a mechanical stimulus to the skin can be encoded in the firing rate of the primary afferent neuron that innervates the skin; the greater the force applied to the skin, the larger the resulting receptor potential in the primary afferent neuron will be and, as a consequence, the faster the rate of action potentials triggered by the receptor potential will be. Research has shown many neurons employ rate coding in the sense that the firing rate of a neuron shows a consistent relationship to particular parameters of sensory stimuli, upcoming movements, or other aspects of behavior.
The amount of information in such rate codes is constrained by several factors. One factor is a neuron’s range of firing rates. The upper limit of this range is set by the maximal frequency that a neuron can fire action potentials, which is determined by the duration of the absolute and relative refractory periods (see
Fig. 5.4 ) and rarely exceeds 1000 Hz. The lower limit of the firing range is, of course, 0 Hz, as neurons cannot fire at negative rates. To avoid this problem, many neurons have spontaneous activity levels. These can be quite high (e.g., some Purkinje cells fire spontaneously at 100 Hz) and let a cell either increase or decrease its activity over a similar range in response to inputs. A second constraining factor is the variability of neuron’s firing rate, which determines the resolution of the neuron’s information coding.
CHAPTER 5 Generation and Conduction of Action Potentials